MySQL's GROUP\_CONCAT function is used to merge multiple rows of data into one row, and is often used to splice the values ??of a column in grouping queries. Its basic syntax is GROUP\_CONCAT([DISTINCT] expr [,expr ...] [ORDER BY ...] [SEPARATOR str\_val]), such as SELECT department, GROUP\_CONCAT(name) AS employees FROM staff GROUP BY department; It can realize the merger of employee names by department. Note when using: 1. The default delimiter is a comma, which can be customized by SEPARATOR; 2. It can be deduplicated through DISTINCT; 3. Use ORDER BY to control the splicing order; 4. The default maximum length is 1024 bytes. The result will be truncated by excessively long. It can be modified by SET SESSION group\_concat\_max\_len; 5. The sorting is invalid and may be due to the wrong position of the statement; 6. The NULL value will not appear in the result and can be handled by IFNULL. Practical applications include scenarios such as querying user roles and merging tags. It is recommended to use indexes reasonably, control splicing lengths, and pay attention to the impact of character sets to improve performance and avoid waste of resources. Mastering these key points can effectively respond to the needs of aggregate display.
MySQL's GROUP_CONCAT function is very useful in actual use, especially when multiple rows of data need to be merged into one row to display. It is usually used in grouping queries, splicing multiple values ??of a certain column into a string.
Basic usage and syntax
The basic syntax of GROUP_CONCAT is as follows:
GROUP_CONCAT([DISTINCT] expr [,expr ...]
[ORDER BY {unsigned_integer | col_name | expr}
[ASC | DESC]]
[SEPARATOR str_val])Common writing methods such as:
SELECT department, GROUP_CONCAT(name) AS employees FROM staff GROUP BY department;
This way, you can group by department and put together the names of all employees in each department to display them.
Note:
- The default separator is a comma
,which can be customized throughSEPARATOR. - If you don't want to repeat the value, you can add
DISTINCT. - The splicing order can be controlled through
ORDER BY.
FAQs and Limitations
The result is truncated?
GROUP_CONCATin MySQL has a length limit, and the default maximum is 1024 bytes. If the string you spelled out is too long, it will be truncated.Solution:
- View the current settings:
SHOW VARIABLES LIKE 'group_concat_max_len'; - Temporary modification (session level):
SET SESSION group_concat_max_len = 1000000; - Permanent modification requires changing
group_concat_max_lenin the configuration file and restarting MySQL.
- View the current settings:
Sorting does not take effect? If
ORDER BYis used but it has no effect, it may be because it is placed in the wrong position. Make sure it follows immediately after the expression.Does it also appear when the splicing field is empty? If the field of a record is
NULL, this value will not appear in the stitching result. But if you want to keep the null value tag, you can handle it in SQL, for example withIFNULL(name, '').
Examples of practical application scenarios
Scenario 1: Query the user's role name
Suppose there is a user role association table and you want to find out the entire role name of each user:
SELECT u.user_id, GROUP_CONCAT(r.role_name SEPARATOR ', ') AS roles FROM users u JOIN user_roles ur ON u.user_id = ur.user_id JOIN roles r ON ur.role_id = r.role_id GROUP BY u.user_id;
This way, you can directly display information like "administrator, edit" on the front end.
Scene 2: Merge the labels that remove heavy weights
For example, in the article tag system, an article may have multiple tags, but I want to avoid repeated display:
SELECT article_id, GROUP_CONCAT(DISTINCT tag ORDER BY tag ASC) AS tags FROM article_tags GROUP BY article_id;
Recommendations and precautions for use
- Don't abuse
GROUP_CONCAT: Although it is convenient, if the amount of spliced ??data is particularly large, it will affect performance, especially when the table data is large and there is no suitable index. - Try to use with indexes : For example, establishing an index on
GROUP BYfield can speed up grouping efficiency. - Reasonably control the splicing length : Set the appropriate
group_concat_max_lenaccording to business needs, which not only ensures availability but also does not waste resources. - Pay attention to the influence of character set : If the splicing content contains Chinese or other multi-byte characters, you should also consider whether the length is sufficient.
Basically that's it. As long as you master the basic usage and common pitfalls, GROUP_CONCAT can help you easily solve many aggregate display needs.
The above is the detailed content of mysql group_concat function. For more information, please follow other related articles on the PHP Chinese website!
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